Lithium Nickel-Manganese-Cobalt Oxide Cathode Powders for High Voltage Lithium-Ion Batteries

ABSTRACT

A lithium metal oxide powder for a cathode material in a rechargeable battery comprises a core and a surface layer. The surface layer is delimited by an outer and an inner interface. The inner interface is in contact with the core. The cathode material has a layered crystal structure comprising the elements Li, M, and oxygen. M has the formula M=(Ni z (Ni 1/2  Mn 1/2 ) y  Co x ) 1-k  A k , with 0.15≤x≤0.30, 0.20≤z≤0.55, x+y+z=1 and 0&lt;k≤0.1. The Li content is stoichiometrically controlled with a molar ratio 0.95≤Li:M≤1.10. A is at least one dopant and comprises Al. The core at the inner interface has an Al content of 0.3-3 mol %. The surface layer comprises an intimate mixture of Ni, Co, Mn, LiF and Al 2 O 3  determined by XPS. The surface layer has a Mn content that decreases from the Mn content at the inner interface to less than 50% of the Mn content at the outer interface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional application of U.S. application Ser.No. 15/544,962, filed Jul. 20, 2017, which is a National Stageapplication of International Application No. PCT/IB2016/050257, filedJan. 20, 2016, and which also claims priority under 35 U.S.C. § 119 toEuropean Application No. 15152289.3, filed Jan. 23, 2015.

TECHNICAL FIELD AND BACKGROUND

This invention relates to improved cathode materials for rechargeablelithium-ion batteries. The cathode material contains Ni, Mn and Co, issurface treated and has a modified composition which shows improvedcycle stability during long term cycling in full cells, especially athigh charge cut-off voltage (>4.35V), at both room and elevatedtemperatures.

Commercially available lithium-ion batteries typically contain agraphite-based anode and cathode materials. A cathode material isusually a powderous material capable to reversibly intercalate andde-intercalate lithium. Historically LiCoO₂ was the dominating cathodematerial for rechargeable lithium batteries. Recently the so-called NMCcathode materials replace LiCoO₂ in many applications. “NMC” is anabbreviation for nickel-manganese-cobalt, and it is used for lithiumtransition metal based oxides, where the transition metal is a mixtureof basically Ni, Mn and Co, having roughly the stoichiometry LiMO₂,where M=Ni_(x)Mn_(y)Co_(z). Additional doping is possible, and typicaldoping elements are Al, Mg, Zr etc. The crystal structure is an orderedrocksalt structure, where the cations order into 2-dimensional Li and Mlayers. The space group is R-3M. There are many different compositionspossible, often categorized and named after their nickel, manganese andcobalt content. Typical NMC based materials are “111” withM=Ni_(1/3)Mn_(1/3)Co_(1/3), “442” with M=Ni_(0.4)Mn_(0.4)Co_(0.2), “532”with M=Ni_(0.5)Mn_(0.3)Co_(0.2), “622” with M=Ni_(0.6)Mn_(0.2)Co_(0.2)etc. It is known that the gravimetric energy density of NMC isincreasing with Ni content for a constant Li potential, for example,NMC622 has a higher gravimetric energy density than NMC532 and NMC111,when charged to the same voltage.

Due to their high energy density, rechargeable lithium and lithium-ionbatteries can be used for a variety of purposes. One of the mostimportant applications is in portable electronic applications, such ascellular phones, laptop computers, digital and video cameras. Anotherother very important application is automotive, including BEV (batteryelectric vehicles), HEV (hybrid electric vehicles) and PHEV (plug-inhybrid electric vehicles).

Regarding portable applications, NMC is not competitive with LiCoO₂ intothe high end portable devices, for example in polymer cells forsmartphones, while successfully replacing LiCoO₂ in low end portabledevices, for example cylinder cells for laptop batteries. A major reasonfor this is that the volumetric energy density of commerciallysuccessful NMC products, which are mainly NMC111 and NMC532, is lowercompared to LiCoO₂, when charged to the normal applicable voltage (4.2V)in polymer cells. There are some advanced high voltage LiCoO₂ productscommercially available in the market, which can give a decent cyclestability in full cells, when cycled up to 4.35 V or even 4.4V. In orderto make NMC competitive with LiCoO₂ concerning high energy density, ahigher application voltage has to be used, for example, 4.35V or 4.4V,and NMC compositions should switch to higher Ni content, for example,from NMC111 to NMC622. Therefore, so-called “high Ni” NMC (with a Nicontent of at least 45 mol %) cathodes that can be cycled stably at ahigh voltage are needed for portable applications.

Regarding automotive applications, NMC is currently dominating themarket, due to its lower cost compared to LiCoO₂. The requirements forbatteries for automotive applications are stricter than those forportable electronics. A very long cycle life is mandatory, usually 80%capacity of the batteries should remain after 2000 cycles (at a 4.2Vcharge), using a full charge and discharge cycle. The high temperaturecycle stability should be good, because batteries in the automotiveusually work at an elevated temperature. There are also very strictrequirements for the other properties of the batteries, for example,calendar life, safety, etc. Similar to batteries for portableapplication, improving the energy density of the cathode is primordialin the automotive applications. Effective approaches may be: using NMCmaterials with a high Ni content, for example NMC622, and increasing thecharge cut-off voltage from currently 4.2V to 4.35V or even 4.4V.

However, there are several issues if one wants to use high Ni NMC athigh voltage. First, high Ni NMC usually has serious issues of solublebase content. The soluble base content refers to the presence of surfaceimpurities like Li₂CO₃ and LiOH. Li₂CO₃ and LiOH could either come fromunreacted reagents of lithium sources, which are usually Li₂CO₃ or LiOH,or from ion exchange reactions with Li present in solvents, which canform LiOH and protons. The soluble base content is usually measured by atechnique called pH titration, as is explained in WO2012/107313. Thesoluble bases will eventually cause a serious gas generation in fullcells, which is usually called “bulging” in full cell tests. Serious gasgeneration/bulging issues will result in bad cycling life of battery,and safety concerns. In WO2011/054441, the authors propose a special LiFcoating layer on NMC cathode materials, which can significantly reducethe soluble base content and suppress the gas generation of NMC polymercells.

Another issue is that it is difficult to achieve a good cycle stabilitywhen NMC is charged to high voltages, for example 4.35V or even 4.4V.The reasons for this failure mechanism of NMC/graphite polymer cellswhen operated at high charge cut-off voltage is still unclear. It isknown, e.g. from U.S. Pat. No. 6,218,048, that one of the main drawbacksof 4V or higher secondary lithium and lithium-ion batteries iselectrolyte decomposition during the charging process or during theshelf life of the battery in its charged state. The negative effects ofthis decomposition are considerably accelerated at elevatedtemperatures. Accordingly, to decrease electrolyte decomposition inconventional cells, low voltage limits are applied strictly during thecell charge process. When manganese-rich and cobalt-rich lithiated metaloxides are used as positive electrode materials, manganese and cobaltdissolution can occur in the cell. This dissolution is observed in theelectrolyte and results in a reduction in the capacity and cycleabilityof the cell. In particular, the negative effect of manganese dissolutionis more pronounced because it is believed that the dissolved manganesecatalyzes electrolyte polymerization and/or decomposition. It is ingeneral needed to limit the dissolution of any transition metal in thepositive electrode into the electrolyte of the battery.

In J. Electrochem. Soc. 2013 160(9): A1451-A1456, Dahn et al. speculatethat the polymer cells using NMC fails due to indissoluble electrolyteoxidized products created near the cathode surface, and moving to theanode side to block the SEI (solid-electrolyte interface), and theneventually blocking the diffusion path of Li⁺. Dahn also proposes thatan effective surface coating on the cathode or effective functionalelectrolyte additives could suppress the electrolyte oxidization andextend the cycle life of the batteries. In US2009/0087362, the authorsprovide a LiCoO₂ powder which is covered by an AlF₃ layer. This coatedLiCoO₂ shows improved cycle stability in a LiCoO₂/Li half cell whencharged to 4.5 V, at both 25° C. and 55° C.

In view of the problems cited before, in order to use high Ni NMCmaterials in high voltage applications, an effective surfacemodification is needed. An object of the present invention is to provideNMC cathode materials with high Ni content that are showing the improvedproperties required for high end portable and automotive applications.

SUMMARY

Viewed from a first aspect, the invention can provide a lithium metaloxide powder for a cathode material in a rechargeable battery,comprising a core and a surface layer, the surface layer being delimitedby an outer and an inner interface, the inner interface being in contactwith the core, the core having a layered crystal structure comprisingthe elements Li, M and oxygen, wherein M has the formula M=(Ni_(z)(N_(1/2) Mn_(1/2))_(y) Co_(x))_(i-k) A_(k), with 0.15≤x≤0.30,0.10≤z≤0.55, x+y+z=1 and 0<k≤0.1, wherein the Li content isstoichiometrically controlled with a molar ratio 0.95≤Li:M≤1.10; whereinA is at least one dopant and comprises Al, wherein the core has an A1content of 0.3-3 mol % and a F content of less than 0.05 mol %; whereinthe surface layer comprises an intimate mixture of Ni, Co, Mn, LiF andAl₂O₃; and wherein the surface layer has an Al content that increasesfrom the Al content of the core at the inner interface to at least 10mol % at the outer interface, and a F content that increases from lessthan 0.05 mol % at the inner interface to at least 3 mol % at the outerinterface, the Al and F contents being determined by XPS. In oneembodiment the Al content in the core is 0.5-2.5 mol %, as determined byXPS. In another embodiment the powder consists of the core and surfacelayer described above.

The invention may also provide a lithium metal oxide powder for acathode material in a rechargeable battery, comprising of a core and asurface layer, the surface layer being delimited by an outer and aninner interface, the inner interface being in contact with the core, thecore having a layered crystal structure comprising the elements Li, Mand oxygen, wherein M has the formula M=(Ni_(z) (N_(1/2) Mn_(1/2))_(y)Co_(x))_(i-k) A_(k), with 0.15≤x≤0.30, 0.10≤z≤0.55, x+y+z=1 and 0<k≤0.1,wherein the Li content is stoichiometrically controlled with a molarratio 0.95≤Li:M≤1.10; wherein A is at least one dopant and comprises Al,wherein the core has an Al content of 0.3-3 mol %; wherein the surfacelayer comprises an intimate mixture of Ni, Co, Mn, LiF and Al₂O₃; andwherein the surface layer has a Mn content that decreases from the Mncontent of the core at the inner interface, to less than 50%, andpreferably less than 45%, of the Mn content of the core at the outerinterface, the A1 content in the core and the Mn content beingdetermined by XPS. In one embodiment the surface layer further has a Nicontent that decreases from the Ni content of the core at the innerinterface, to less than 25%, and preferably less than 20% of the Nicontent of the core at the outer interface, as determined by XPS. Inanother embodiment, the surface layer further has a Co content thatdecreases from the Co content of the core at the inner interface, toless than 35%, and preferably less than 25% of the Co content of thecore at the outer interface, as determined by XPS. It is well understoodthat the contents of Mn, Co and Ni have a constant value in the core ofthe material. The invention can also provide a lithium metal oxidepowder that has both the features of the A1 and the F gradient, and alsothe Mn gradient described before. In another embodiment the powderconsists of the core and surface layer described above.

The composition of the core, i.e. the indices x, y, z and k aredetermined by the stoichiometry of the elements constituting M assupplied in the precursors of these elements, and can be checked byknown analysis methods, such as ICP. In the previous embodiments the A1content in M is preferably between 0.5 and 2 mol %, corresponding to0.005≤k≤0.02, the lower limit being the guarantee that the desiredproduct advantages are obtained, the upper limit indicating that asurplus of A1 is not really needed to achieve the advantages. In anotherembodiment, A=Al and Ca, with 0.005≤k≤0.02. In the different productembodiments, the F content is preferably equal to 0 mol % in the core ofthe oxide powder. In the various embodiments also, the thickness of thesurface layer may be more than 50 nm and less than 400 nm. The thicknessof this surface layer is more than 50 nm, preferably more than 150 nm;and less than 400 nm, preferably less than 200 nm. It is clear that theouter interface of the surface layer corresponds to the actual surfaceof the particle. The inner interface may also be defined as the depthestablished with XPS where the A1 content is at least 0.05 mol % higherthan the constant doping level in the core of the material, alsomeasured with XPS. If the surface layer thickness is less than 50 nm, itmay be that the layer does not effectively reduce the content of thesoluble bases and limit the dissolution of Mn in the electrolyte. If thelayer is thicker than 400 nm, it may be that the intercalation andde-intercalation of Li is hindered too much, and the specific capacityof the powder is then lowered.

The thickness of the surface layer is determined by XPS measurement. Asputtering rate in SiO₂: 6.0 nm/minute is applied to calculate thedepth/thickness. The thickness here is obtained by the sputtering timemultiplied by the (reference) sputtering rate in SiO₂. During the XPSmeasurement, it is difficult to obtain the sputtering rate of measuredobjectives. A typical way is to normalize the thickness by using astandard sputtering rate (in SiO₂ here) for all samples. Therefore, itis not necessary true that the thickness calculated here is the same ascould be obtained by other spectra methods, for example, ScanningElectron Microscopy (SEM). However, for descriptions of the propertiesof the coating layer, such as element distribution with different layerthickness, XPS can provide accurate qualitative and quantitative data.

In an embodiment of the product of the invention described before, thesurface layer consists of an intimate mixture of elements of the core,LiF and Al₂O₃, and further contains either one or more compounds fromthe group consisting of CaO, TiO₂, MgO, WO₃, ZrO₂, Cr₂O₃ and V₂O₅. In aparticular embodiment the surface layer consists of an intimate mixtureof elements of the core, LiF and either nanometric crystalline Al₂O₃, ornanometric crystalline Al₂O₃ and sub-micrometric CaO.

In an embodiment, the F content of the core may be equal to 0 mol %. Indifferent embodiments of the present invention, the lithium metal oxidepowder has one or more of the following characteristics:

a) 0.20≤z≤0.55.

b) 0.15≤x≤0.20, 0.40≤z≤0.55 and 1≤Li:M≤1.10.

c) A=Al or A=Al and Ca, and 0.005≤k≤0.02.

d) A=Al or A=Al and Ca, k=0.01±0.005, x=0.20±0.02, y=0.40±0.05,z=0.40±0.05 and 1≤Li:M≤1.10.

It is clear that further product embodiments according to the inventionmay be provided by combining features that are covered by the differentproduct embodiments described before.

Viewed from a second aspect, the invention can provide a method formaking the lithium metal oxide powder according to the invention,comprising the steps of:

-   -   providing a first mixture comprising a lithium M′-oxide powder,        with M′=Ni_(z) (N_(1/2) Mn_(1/2))_(y) Co_(x), 0.15≤x≤0.30,        0.10≤z≤0.55 and x+y+z=1, and a first source of A comprising Al,    -   heating the first mixture to a first sintering temperature of at        least 500° C.,    -   sintering the first mixture at the first sintering temperature        for a first period of time,    -   cooling the first sintered mixture, preferably down to room        temperature,    -   adding a fluorine-containing polymer and a second source of A        comprising A1 to the mixture of the sintered mixture, thereby        obtaining a second mixture,    -   heating the second mixture to a second sintering temperature        between 250 and 500° C., and    -   sintering the second mixture at the second sintering temperature        for a second period of time, thereby obtaining the lithium metal        oxide powder, and cooling the powder. In one embodiment        0.20≤z≤0.55. In different embodiments, the first sintering        temperature is between 650 and 750° C., and the second sintering        temperature is between 350 and 400° C. These temperature ranges        proved to be effective for achieving the desired product        properties. In one embodiment, both the first period of time of        the first sintering step and the second period of time of the        second sintering step are between 5 and 10 hr. In another        embodiment, either one or both of the first and the second        source of A is Al₂O₃. There may also be added CaO to A. In this        embodiment either one or both of the first and the second source        of A may further comprise either one or more compounds selected        from the group consisting of CaO, TiO₂, MgO, WO₃, ZrO₂, Cr₂O₃        and V₂O₅. In another embodiment the source of A comprises a        nanometric alumina powder having a D50<100 nm and a BET≥50 m²/g.        This source may also comprise a sub-micrometric CaO powder        having a D50<200 nm and a BET≥30 m²/g. During the second        sintering step, the crystalline structure of the alumina that is        added is preserved in the final product, which is advantageous        for obtaining the desired product properties. In still another        embodiment, the amount of fluorine-containing polymer in the        second mixture is between 0.1 and 2 wt %, and preferably between        0.2 and 0.5 wt %. In different embodiments, the        fluorine-containing polymer is a PVDF homopolymer, or a PVDF        copolymer, or a PVDF-HFP (hexa-fluoro propylene) polymer, or a        PTFE (polytetrafluoroethylene) polymer. It is clear that further        method embodiments according to the invention may be provided by        combining features that are covered by the different method        embodiments described before. Viewed from a third aspect, the        invention can provide an electrochemical cell (such as a Li-ion        battery) comprising a cathode material comprising the lithium        metal oxide powder according to the invention, wherein the        electrochemical cell is used in a portable electronic device,        such as a portable computer, a tablet, a mobile phone, and in an        electrically powered vehicle or an energy storage system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Al and F atomic concentration vs thickness measured by XPS forExample 1

FIG. 2A: Ratio of Mn atomic concentration at the surface (XPS depth=0)vs. Mn atomic concentration at XPS depth=200 nm.

FIG. 2B: Ratio of Ni atomic concentration at the surface (XPS depth=0)vs. Ni atomic concentration at XPS depth=200 nm.

FIG. 2C: Ratio of Co atomic concentration at the surface (XPS depth=0)vs. Co atomic concentration at XPS depth=200 nm.

FIG. 3: Full cell cycle stability of Example 1 between 3.0-4.35V at 25°C.

FIG. 4: Full cell cycle stability of Example 1 between 3.0-4.35V at 45°C.

FIG. 5: Comparison of full cell cycle stability of Example 1 and Example4 vs. Counterexample 1 between 3.0-4.4V at 25° C.

FIG. 6: Comparison of full cell cycle stability of Example 1 and Example4 vs. Counterexample 1 between 3.0-4.4V at 45° C.

FIG. 7: Comparison of full cell cycle stability of Examples 1˜3 vs.Counterexamples between 3.0-4.35V at 25° C.

FIG. 8: Comparison of full cell cycle stability of Examples 1˜3 vs.Counterexamples between 3.0-4.35V at 45° C.

FIG. 9: Full cell thickness increase ratio of Examples 1˜4 vs.Counterexamples after bulging test.

FIG. 10: Comparison of full cell cycle stability of Example 1 vs.Counterexample 6 and 7

DETAILED DESCRIPTION

The invention provides cathode material powders which have an improvedcycle stability when charged up to 4.35V and/or 4.4V in full cells, atboth room and elevated temperature. These materials have a high Nicontent (i.e. at least 45 mol % and at most 70 mol % of the transitionmetal content) which can provide a significantly higher energy densitycompared to existing commercial NMC cathode materials, for exampleNMC111. The powders could even be competitive with commercial LiCoO₂when considering the energy density. Therefore, the cathode materialsaccording to the invention are promising candidates for a use in highend portable electronics and automotive applications.

The authors discovered that NMC cathode powders with surface layers thathave either both an A1 and a fluor gradient in the surface layer, or amanganese gradient in the surface layer have superior characteristicswhen used in Li-ion batteries. The existence of an A1 gradient and a Mngradient in the surface layer may help to improve the cycle stabilitywhen the cathode materials are charged to high voltage (4.35V or 4.4V).The F gradient in the coating layer on the other hand may help to reducethe amount of soluble base and eventually improve the bulging propertiesof a full cell.

In accordance with the invention, the particles forming the powder ofthe invention have a core and a surface layer that may be a coatinglayer. The surface layer is delimited by an outer and an innerinterface, the inner interface being in contact with the core. The coremay have an A1 content more than 0.3 mol % but less than 3.0 mol %, anda F content less than 0.05 mol %, as determined by XPS.

In the first embodiment, the surface layer has an A1 content thatincreases continuously from the A1 content of the core at the innerinterface to more than 10 mol % at the outer interface, and preferablymore than 12 mol %; and has a F content that increases continuously fromless than 0.05 mol % at the inner interface to at least 3 mol % at theouter interface, preferably at least 5 mol % at the outer interface. Theconcentration of the different elements in the surface layer—being atleast Ni, Co, Mn, LiF and —Al₂O₃— and the outer part of the core can bedetermined using X-ray photoelectron spectroscopy (XPS).

In a different embodiment, the surface layer has a Mn content thatdecreases continuously from the Mn content of the core at the innerinterface to less than 50% of the Mn content of the core at the outerinterface, preferably less than 45% of the Mn content of the core at theouter interface. By limiting the Mn content in the surface layer, thedissolution of manganese may be effectively limited. It should be notedthat in US2013/0122370 there is provided a cathode active material forlithium secondary battery containing the compoundLi_(a)Ni_(x)Co_(y)M′_(z)Mn_((i-x-y-z))O₂ which is further doped orcoated with phosphate fluoride, wherein M′ is selected from the groupconsisting of Ca, Mg, Al, Ti, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, B, anda combination thereof; and 0.4<a≤1.3, 0≤x≤0.8, 0≤y≤0.33, 0≤z≤0.33, and0≤x+y+z≤1.

The invention may further provide a cathode active material comprising:a composite oxide particle containing at least lithium Li, Ni, Mn andcobalt Co; and a surface layer which is provided on at least a part ofthe composite oxide particle and has a compound containing Li and anelement of at least one of aluminum Al, manganese Mn, and fluor F,wherein a ratio [Al(T)Ni(S)/Al(S)Ni(T)] of an atomic ratio [Al(T)/Ni(T)]of Al to Ni as an average of the whole cathode active material to anatomic ratio [Al(S)/Ni(S)] of Al to Ni in the surface layer of thecathode active material is larger than a ratio [F(T)Ni(S)/F(S)Ni(T)] ofan atomic ratio [F(T)/Ni(T)] of F to Ni as an average of the wholecathode active material to an atomic ratio [F(S)/Ni(S)] of F to Ni inthe surface layer of the cathode active material. The invention may alsoprovide a cathode active material comprising: a composite oxide particlecontaining at least lithium Li, Ni, Mn and cobalt Co; and a surfacelayer which is provided on at least a part of the composite oxideparticle and has a compound containing lithium Li and an element of atleast one of aluminum Al, manganese Mn, and fluor F, wherein a ratio[Mn(T)Ni(S)/Mn(S)Ni(T)] of an atomic ratio [Mn(T)/Ni(T)] of Mn to Ni asan average of the whole cathode active material to an atomic ratio[Mn(S)/Ni(S)] of Mn to Ni in a surface layer of the cathode activematerial is smaller than a ratio [Al(T)/Ni(S)/Al(S)Ni(T)] of an atomicratio [Al(T)/Ni(T)] of Al to Ni as an average of the whole cathodeactive material to an atomic ratio [Al(S)/Ni(S)] of Al to Ni in thesurface layer of the cathode active material. The Al, Mn, Ni and Fcontents may be determined by XPS.

The invention also provides a process as described in the summary. Thefirst mixture is obtained by blending a lithium transition metal oxidecore powder and a first source of A that comprises Al. For preparingthis core powder, known methods are used. For example, lithium carbonateand a mixed Ni—Mn—Co oxy-hydroxide are homogeneously blended a verticalsingle-shaft mixer by a dry powder mixing process. The blend ratio maybe targeted to obtain the composition of the oxide powder without A andF. This dry powder mixture is sintered in a tunnel furnace in anoxidizing atmosphere. The sintering temperature is >850° C. and thedwell time is ˜10 hrs. Dry air is used as an oxidizing gas. The processused for preparing the core powder yields particles that have ahomogeneous composition, resulting in a constant Ni, Mn and Co contentin the core.

In a preferred embodiment of the inventive method, A is least one dopantand comprises Al. A can be, besides Al, one or more elements from thegroup of Ca, Mg, Zr, W, Ti, Cr and V. A dopant, also called a dopingagent, is a trace impurity element that is inserted into a substance (invery low concentrations) in order to alter the electrical properties orthe optical properties of the substance. The source of A is preferably acompound selected from the group consisting of a metal oxide, forexample—besides Al₂O₃—CaO, TiO₂, MgO, WO₃, ZrO₂, Cr₂O₃, V₂O₅ andmixtures thereof, more particularly a mixture of Al₂O₃ and CaO. Theexamples show that the combination of Al₂O₃ and CaO is particularlyefficient. The preferred source of A1 is a nanometric alumina powder,for example fumed alumina. The alumina can be obtained by precipitation,spray drying, milling, etc.

In one embodiment the alumina typically has a BET of at least 50 m²/gand consists of primary particles having a D50<100 nm, the primaryparticles being non-aggregated. In another embodiment fumed alumina orsurface treated fumed alumina is used. Fumed alumina nanoparticles areproduced in high temperature hydrogen-air flames and are used in severalapplications that involve products of everyday use. The preferred sourceof Ca is a sub-micrometric calcium oxide powder.

In one embodiment the CaO typically has a BET of at least 30 m²/g andconsists of primary particles having a D50<200 nm, the primary particlesbeing non-aggregated.

In accordance with the invention and process described in the summary,in the first heating step, the first mixture is heated to a temperature(referred to as first sintering temperature) which is at least 500° C.,preferably at least 600° C., and more preferably at least 650° C.Preferably, the first sintering temperature is at most 800° C., morepreferably at most 750° C., most preferably at most 700° C. Theselection of this sintering temperature is important to obtain thedoping of the lithium metal oxide core by element A. The first sinteringtime is the period of heat treatment at the constant sinteringtemperature. The sintering time is preferably at least 3 hours, morepreferably at least 5 hours. Preferably, the sintering time is less than15 hours, more preferably less than 10 hours.

After the first sintering step, the obtained powder is mixed with asource of F being a fluorine-containing polymer. A typical example forsuch a polymer is a PVDF homopolymer or PVDF copolymer (such as HYLAR®or SOLEF® PVDF, both from Solvay SA, Belgium). Another known PVDF basedcopolymer is for example a PVDF-HFP (hexa-fluoro propylene). Suchpolymers are often known under the name “Kynar®”. Teflon—or PTFE—couldalso be used as polymer. The source of A in the second step can be thesame as for the first step: a compound selected from the groupconsisting of a metal oxide, for example—besides Al₂O₃—TiO₂, MgO, WO₃,ZrO₂, Cr₂O₃, V₂O₅ and mixtures thereof. The preferred source of A1 is ananometric alumina powder, for example fumed alumina.

For the second sintering step, the second sintering temperature of themixture is at least 250° C., preferably at least 350° C. Also, thesecond sintering temperature is preferably at most 500° C., morepreferably less than 400° C. The selection of this sintering temperatureis important to obtain a surface layer that actually is a coatingcomprising the dopant A (at least Al) and fluor. The second sinteringtime is preferably at least 3 hours, more preferably at least 5 hours.Preferably, the sintering time is less than 15 hours, more preferablyless than 10 hours.

In the second sintering step, due to the lower sintering temperature,the crystalline structure of the fumed alumina is maintained during thecoating process and is found in the coating layer surrounding thelithium metal oxide core. Also in the second sintering step, thefluorine-containing polymer—which is free of Li—starts to decompose incontact with the core material, as is described in WO2011/054441. Thepolymer is completely decomposed and lithium fluoride is formed, whichis found in the surface layer of the particles. The LiF originates fromthe reaction of the decomposing polymer with lithium containing surfacebase of the lithium transition metal oxides. Whereas a normal fluoridecontaining polymer just melts upon heating, it can be established thatthe contact with the Li (soluble) base on the surface of the transitionmetal oxide initiates a chemical reaction leading to the decompositionof the polymer. It can be speculated that the LiF film protects the Liin the particle, thus preventing it from reacting with carbon to formLi₂CO₃. The obtained surface layer has the following function: the thinlayer comprising LiF replaces the reactive surface base layer, thusreducing the base content practically to zero at the core's surface, andimproves the overall safety.

The invention will now be illustrated in the following Examples:

Experimental Tests Used in the Examples

a) Full Cell Making

a.1) Slurry Making and Coating

A slurry is prepared by mixing 700 g of NMC cathode material with NMP,47.19 g of super P® (conductive carbon black of Timcal) and 393.26 g of10 wt % PVDF based binder in NMP solution. The mixture is mixed for 2.5hrs in a planetary mixer. During mixing additional NMP is added. Themixture is transferred to a Disper mixer and mixed for 1.5 hrs underfurther NMP addition. A typical total amount of NMP used is 423.57 g.The final solid content in the slurry is about 65 wt %. The slurry istransferred to a coating line. Double coated electrodes are prepared.The electrode surface is smooth. The electrode loading is 9.6 mg/cm².The electrodes are compacted by a roll press to achieve an electrodedensity of about 3.2 g/cm³. The electrodes are used to prepare pouchcell type full cells as described hereafter.

a.2) Full Cell Assembly

For full cell testing purposes, the prepared positive electrodes(cathode) are assembled with a negative electrode (anode) which istypically a graphite type carbon, and a porous electrically insulatingmembrane (separator). The full cell is prepared by the following majorsteps: (a) electrode slitting, (b) electrode drying, (c) jellyrollwinding, and (d) packaging.

(a) electrode slitting: after NMP coating the electrode active materialmight be slit by a slitting machine. The width and length of theelectrode are determined according to the battery application.

(b) attaching the taps: there are two kinds of taps. Aluminum taps areattached to the positive electrode (cathode), and copper taps areattached to the negative electrode (anode).

(c) electrode drying: the prepared positive electrode (cathode) andnegative electrode (anode) are dried at 85° C. to 120° C. for 8 hrs in avacuum oven.

(d) jellyroll winding: after drying the electrode a jellyroll is madeusing a winding machine. A jellyroll consists of at least a negativeelectrode (anode) a porous electrically insulating membrane (separator)and a positive electrode (cathode).

(e) packaging: the prepared jellyroll is incorporated in a 650 mAh cellwith an aluminum laminate film package, resulting in a pouch cell.Further, the jellyroll is impregnated with the electrolyte. Theelectrolyte used is a commercial product from Panax Etec Ltd. Thecomposition is 1 M LiPF₆ in EC:DEC:EMC (1:1:1, m/m/m) with VC, LiBOB andPRS as additives. The quantity of electrolyte is calculated inaccordance with the porosity and dimensions of the positive and negativeelectrode, and the porous separator. Finally, the packaged full cell issealed by a sealing machine.

b) Full Cell Cycling

The full cell is cycled at both 25° C. (=RT) and 45° C. (=HT) usingToscat-3100 computer-controlled galvanostatic cycling stations (Toyo)between 3.0V and 4.35V or 4.4V under CC/CV (constant current/constantvoltage) mode at 1 C rate (corresponding to the current which dischargesa charged cell within 1 hr). In the cycling stability test there ismeasured up to which cycle No. at least 80% of the initial capacityremains.

c) Full Cell Bulging Test

The fully charged cells are stored in an oven at 90° C. for 4 hours. Thereaction between active material and electrolyte generates gas in a fullcell, resulting in the increase of battery thickness (bulging). Thethickness of the full cells is measured before and after storing in theoven. The reported value is the ratio of increased full cell thickness,expressed in % increase versus the initial thickness.

d) XPS Measurement

The measurements are carried out in a Quantera SXM™ from ULVAC-PHI (Q2).The measurements are performed using monochromatic Al—Kα-radiation and aspot size of 100 μm scanning across an area of 1200×500 μm (HighSensitivity Mode). The measurement angle θ is 45°; at this setting theinformation depth is approximately 7 nm. By means of wide-scanmeasurements the elements present at the surface are identified.Accurate narrow-scans are performed to determine the precise surfacecomposition. Concentration—depth profiles are determined by alternatingmeasurements and ion bombardment (Argon ions, Vi=4 kV, raster 3×3 mm,sputter rate in SiO₂: 6.0 nm/minute). The XPS gives a measurement onlyfrom the surface up to approx. 200 nm inside the particles. Knowntechniques such as ICP give the average composition of the powder. It isknown that ICP gives a more accurate average measurement than XPS, butXPS is especially adequate to investigate the differences in compositionat different depths in a surface layer.

Example 1: A powder according to the invention is manufactured on apilot line of Umicore (Korea), by the following steps:

(a) Blending of lithium and nickel-manganese-cobalt precursor: lithiumcarbonate and a mixed Ni—Mn—Co oxy-hydroxide are homogeneously blendedin a vertical single-shaft mixer by a dry powder mixing process. Theblend ratio is targeted to obtain Li_(1.01) (Ni_(0.4) (N_(1/2)Mn_(1/2))_(0.4) Co_(0.2))_(0.99) O₂, which can be easily verified by ananalysis technique such as ICP.

(b) Synthesizing in an oxidizing atmosphere: the powder mixture fromstep (a) is sintered in a tunnel furnace in an oxidizing atmosphere. Thesintering temperature is >900° C. and the dwell time is ˜10 hrs. Dry airis used as an oxidizing gas.

(c) Milling: after sintering, the sample is milled in a grinding machineto a particle size distribution with D50=11-12 μm. The span is 1.20.Span is defined as (D90−D10)/D50 where DXX are the corresponding XXvalues of the volume distribution of the particle size analysis.

(d) one step A1 doping and alumina coating: 1 kg of the Li_(1.01)(Ni_(0.4) (Ni_(1/2) Mn_(1/2))_(0.4) Co_(0.2))_(0.99) O₂ powder from step(c) is filled into a mixer (in the example a 2 L Henschel type Mixer)and 2 g of fumed alumina (Al₂O₃) nano-powder is added as well. Afterhomogeneously mixing (usually 30 mins at 1000 rpm), the mixture issintered in a box furnace in an oxidizing atmosphere. The sinteringtemperature is 700° C. and the dwell time is ˜5 hrs. Dry air is used asan oxidizing gas. It can be verified that after the sintering step atthis temperature A1 is doped in the lithium metal oxide (core), and XPSmeasurements show a gradient that is established at the surface withincreasing A1 content, whereas the surface itself is covered with a verythin Al₂O₃ coating. After this step the material could be represented bythe overall formula Li_(1.01) ((Ni_(0.4) (N_(1/2) Mn_(1/2))_(0.4)Co_(0.2))_(0.996) Al_(0.004))_(0.99) O₂.

(e) Alumina and LiF coating: 1 kg of powder obtained from process (d) isfilled into a mixer (in the example a 2 L Henschel type Mixer), 2 g offumed alumina (Al₂O₃) nano-powder and 3 g polyvinylidene fluoride (PVDF)powder is added as well. After homogeneously mixing (usually 30 mins at1000 rpm), the mixture is sintered in a box furnace in an oxidizingatmosphere. The sintering temperature is 375° C. and the dwell time is˜5 hrs. Dry air is used as an oxidizing gas. The surface layerestablished in step (d) is not creating a barrier for the PVDF to reactwith Li present at the inner surface, and to form LiF. It can beverified that after the second sintering step the surface layer is amixture of elements of the core, LiF and Al₂O₃. The final A1 content is0.8 mol % (as can be determined by ICP).

Example 2: A powder according to the invention is manufactured on apilot line of Umicore (Korea), by the following steps:

steps (a), (b) and (c) are identical to Example 1, followed by:

(d) one step A1 doping and alumina coating: 1 kg of the Li_(1.01)(Ni_(0.4) (N_(1/2) Mn_(1/2))_(0.4) Co_(0.2))_(0.99) O₂ powder from step(c) is filled into a mixer (in the example a 2 L Henschel type Mixer)and 2 g of fumed alumina (Al₂O₃) nano-powder is added as well. Afterhomogeneously mixing (usually 30 mins at 1000 rpm), the mixture issintered in a box furnace in an oxidizing atmosphere. The sinteringtemperature is 500° C. and the dwell time is ˜10 hrs. Dry air is used asan oxidizing gas. It can be verified that after the sintering step atthis temperature A1 is doped in the lithium metal oxide (core), and XPSmeasurements show a gradient that is established at the surface withincreasing A1 content, whereas the surface itself is covered with a verythin Al₂O₃ coating. After this step the material could be represented bythe overall formula Li_(1.01) ((Ni_(0.4) (N_(1/2) Mn_(1/2))_(0.4)Co_(0.2))_(0.996) Al_(0.004))_(0.99) O₂.

step (e) Alumina and LiF coating: is identical as in Example 1

Example 3: A powder according to the invention is manufactured on apilot line of Umicore (Korea), by the following steps:

-   -   steps (a), (b) and (c) are identical to Example 1, followed by:    -   (d) one step A1 doping and alumina coating: 1 kg of the        Li_(1.01) (Ni_(0.4) (Ni_(1/2) Mn_(1/2))_(0.4) Co_(0.2))_(0.99)        O₂ powder from step (c) is filled into a mixer (in the example a        2 L Henschel type Mixer) and 1 g of fumed alumina (Al₂O₃)        nano-powder is added as well. After homogeneously mixing        (usually 30 mins at 1000 rpm), the mixture is sintered in a box        furnace in an oxidizing atmosphere. The sintering temperature is        500° C. and the dwell time is ˜10 hrs. Dry air is used as an        oxidizing gas. It can be verified that after the sintering step        at this temperature A1 is doped in the lithium metal oxide        (core), and XPS measurements show a gradient that is established        at the surface with increasing A1 content, whereas the surface        itself is covered with a very thin Al₂O₃ coating. After this        step the material could be represented by the overall formula        Li_(1.01) ((Ni_(0.4) (N_(1/2) Mn_(1/2))_(0.4) Co_(0.2))_(0.998)        Al_(0.002))_(0.99) O₂.    -   (e) Alumina and LiF coating: 1 kg of powder obtained from        process (d) is filled into a mixer (in the example a 2 L        Henschel type Mixer), 4 g of fumed alumina (Al₂O₃) nano-powder        and 3 g polyvinylidene fluoride (PVDF) powder is added as well.        After homogeneously mixing (usually 30 mins at 1000 rpm), the        mixture is sintered in a box furnace in an oxidizing atmosphere.        The sintering temperature is 400° C. and the dwell time is ˜5        hrs. Dry air is used as an oxidizing gas. The surface layer        established in step (d) is not creating a barrier for the PVDF        to react with Li present at the inner surface, and to form LiF.        It can be verified that after the second sintering step the        surface layer is a mixture of elements of the core, LiF and        Al₂O₃. The final A1 content is 1.2 mol % (as can be determined        by ICP).

Example 4: A powder according to the invention is manufactured on apilot line of Umicore (Korea), by the following steps:

(a) Blending of lithium, calcium and nickel-manganese-cobalt precursor:lithium carbonate, CaO and a mixed Ni—Mn—Co oxy-hydroxide arehomogeneously blended in a vertical single-shaft mixer by a dry powdermixing process. The blend ratio is targeted to obtain

Li_(1.01) ((Ni_(0.4) (N_(1/2) Mn_(1/2))_(0.4) Co_(0.2))_(0.995)Ca_(0.005))_(0.99) O₂, which can be easily verified by an analysistechnique such as ICP.

steps (b) and (c) are identical to Example 1, and are followed by:

(d) one step A1 doping and alumina coating: 1 kg of the powder from step(c) is filled into a mixer (in the example a 2 L Henschel type Mixer)and 2 g of fumed alumina (Al₂O₃) nano-powder is added as well. Afterhomogeneously mixing (usually 30 mins at 1000 rpm), the mixture issintered in a box furnace in an oxidizing atmosphere. The sinteringtemperature is 500° C. and the dwell time is ˜10 hrs. Dry air is used asan oxidizing gas. It can be verified that after the sintering step atthis temperature A1 is doped in the lithium metal oxide (core), and XPSmeasurements show a gradient that is established at the surface withincreasing A1 content, whereas the surface itself is covered with a verythin Al₂O₃ coating. After this step the material could be represented bythe formula Li_(1.01) ((Ni_(0.4) (N_(1/2) Mn_(1/2))_(0.4)Co_(0.2))_(0.991) Ca_(0.005) Al_(0.004))_(0.99) O₂.

(e) Alumina and LiF coating: 1 kg of powder obtained from process (d) isfilled into a mixer (in the example a 2 L Henschel type Mixer), 2 g offumed alumina (Al₂O₃) nano-powder and 3 g polyvinylidene fluoride (PVDF)powder is added as well. After homogeneously mixing (usually 30 mins at1000 rpm), the mixture is sintered in a box furnace in an oxidizingatmosphere. The sintering temperature is 400° C. and the dwell time is˜5 hrs. Dry air is used as an oxidizing gas. The surface layerestablished in step (d) is not creating a barrier for the PVDF to reactwith Fi present at the inner surface, and to form LiF. It can beverified that after the second sintering step the surface layer is amixture of elements of the core, LiF and Al₂O₃. The final A1 content is0.8 mol % (as can be determined by ICP).

Counterexample 1: A positive electrode material Li_(1.01) ((Ni_(0.4)(N_(1/2) Mn_(1/2))_(0.4) Co_(0.2))_(0.996) Al_(0.004))_(0.99) O₂ ismanufactured through the process steps (a), (b), (c) and (d), withoutadditional alumina and LiF coating (process (e)).

Counterexample 2: A positive electrode material is manufactured throughthe process steps (a), (b), (c) and (e), where A1 and the polymer wereonly added in the process of step (e) resulting in a final A1 content of0.4 mol % (determined by ICP). The powder after step (b) has the formulaLi_(1.01) (Ni_(0.4) (N_(1/2) Mn_(1/2))_(0.4) Co₀₂)_(0.99) O₂. There isno A1 doping/alumina coating that is typical for process step (d). Theobtained powder thus has no A1 doped in the core.

Counterexample 3: A coated positive electrode material that can berepresented by the formula Li_(1.01) ((Ni_(0.4) (Ni_(1/2)Mn_(1/2))_(0.4) Co_(0.2))_(0.996) Al_(0.004))_(0.99) O₂ is manufacturedthrough the process (a), (b), (c) and (d), without alumina and LiFcoating (process (e)). However, the sintering temp in step (d) is 375°C., resulting in a coating with only alumina instead of a doping withAl, as described in Wu et al., “High Capacity, Surface-Modified LayeredLi[Li_((1-x)/3)Mn_((2-x)/3)Ni_(x/3) Co_(x/3)]O₂ Cathodes with LowIrreversible Capacity Loss,” Electrochemical and Solid State Letters, 9(5) A221-A224 (2006). The obtained powder thus has no Al doped in thecore.

Counterexample 4: A positive electrode material that could berepresented by the formula Li_(1.01) ((Ni_(0.4) (Ni_(1/2)Mn_(1/2))_(0.4) Co_(0.2))_(0.996) Al_(0.004))_(0.99) O_(1.991)F_(0.009)is manufactured through the process (a), (b), (c) and (e). However, theF source is AlF₃, and as is known from US2011/111298 the AlF₃ heated atthe same temperatures as in the present invention (in step (e)) does notreact with the Li at the inner interface of the surface layer. Theobtained powder also has no Al doped in the core.

Counterexample 5: A positive electrode material “NMC622” Li_(1.01)((Ni_(0.4) (Ni_(1/2) Mn_(1/2))_(0.4) Co_(0.2))_(0.99) O₂ is manufacturedthrough the process (a), (b), (c) without any further treatment.

Counterexample 6: A commercial positive electrode material “NMC532”Li_(1.01) (Ni_(0.5) Mn_(0.3) Co_(0.2))_(0.99) O₂ is supplied.

Counterexample 7: A commercial positive electrode material LiCoO₂ issupplied.

Discussion

FIG. 1 and FIGS. 2A-2C show the properties of the surface layer ofExample 1. The gradients of Al and F in the surface layer and the outerportion of the core as measured by XPS are shown in FIG. 1.

There is also a clear effect of suppression of the Mn, Ni and Coconcentrations at the outer interface of the surface, as shown in FIGS.2A-2C (resp. showing the ratio of resp. Mn, Ni and Co (XPS measurements)at the particle surface versus the content at 200 nm depth, whichcorresponds to the outer portion of the core). Such unique surfaceproperties are related to an exceptional electrochemical performance ofthe invented cathode materials. The full cell cycle stability of Example1 between 3.0-4.35V at 25° C. and 45° C. is shown in FIGS. 3 and 4(expressed in % of capacity vs. first cycle=100%). The comparison of thefull cell cycle stability of Example 1 and Example 4 vs. Counterexample1 between 3.0-4.4V at 25° C. and 45° C. is shown in FIGS. 5 and 6, wherethe full cell of Example 1 and Example 4 in each have a higher capacityafter 450 cycles. FIG. 7 gives a comparison of the full cell cyclestability of Example 1-3 vs. Counterexamples 1-5 between 3.0-4.35V at25° C., in FIG. 8 the same is shown for cycling between 3.0-4.35V at 45°C. FIGS. 7 and 8 show that although the cycling stability of Examples1-3 and Counterexample 1 are comparable at room temperature, at hightemperatures (45° C. being characteristic for automotive applications)Examples 1-3 are superior, whereas the inverse is shown for thecomparison between Examples 1-3 and Counterexample 4. Only Examples 1-3are superior in cycling stability at both room and high temperature. InFIG. 7 the top line is for Counterex. 1, and just below is Example 2,and then following are Example 3 and Example 1. In FIG. 8 the top lineat 600 cycles is for Example 3, and just below is Counterexample 4, andthen following is Example 1. In both figures the other lines can bedistinguished using the data of Table 1. Example 4 is not tested at 4.35V but tested at a more extreme condition, which is 4.4V.

Table 1 summarizes the cycle stability of full cells of the differentExamples. When a full cell charged to 4.35V, which is a tough conditionfor ordinary polymer cells using an NMC cathode, Example 1 surprisinglyshows a good cycling stability at both room and elevated temperatures.Even when charged to 4.4V, Example 1 shows a superior cycle stability.The same performance could be achieved in Example 4. The 4.4V cycle testis only applied for Counterexample 1, due to its comparable performancewith Example 1 at 4.35 V cycling. However, the full cell ofCounterexample 1 dies just after 200 cycles.

TABLE 1 Comparison of cycle stability at different cycle conditions RTcycle HT cycle RT cycle HT cycle (4.35 V) (4.35 V) (4.4 V) (4.4 V)Example 1 >1000 cy # >600 cy # >500 cy # >500 cy # Example 2 >1000 cy# >600 cy # NT NT Example 3 >1000 cy # >500 cy # NT NT Example 4 NTNT >500 cy # >500 cy # Counterexample 1 >1000 cy # 500 cy #  200 cy # 200 cy # Counterexample 2 700 cy # 500 cy # NT NT Counterexample 3 300cy # 300 cy # NT NT Counterexample 4 600 cy # >600 cy # NT NTCounterexample 5 200 cy # 150 cy # NT NT (Cycle No. refers to the cyclewith 80% capacity remaining) NT = not tested; cy # = number of cycles

FIG. 9 shows the full cell thickness increase of Examples 1˜4 comparedto the different Counterexamples after a bulging test. Examples 1˜4generally show a lower thickness increase compared to the othermaterials. As was discussed in the background section, the issue of“bulging” of high Ni NMC materials at high voltage is related to badcycling life and safety issues. The bulging problem can be greatlyimproved by the novel surface modification provided in this invention.

FIG. 10 shows a comparison of the specific capacity of Example 1 chargedto 4.35 V with commercial NMC532 charged to 4.2V and commercial LiCoO₂charged to 4.4V. Example 1 can be written as coated NMC622. It is clearthat Example 1 shows a gain of 13% in specific capacity compared toNMC532 and 5% improvement compared to LiCoO₂. The cycling stability ofthe powder according to the invention is equal or even better thanNMC532 and especially LiCoO₂. Such material is an ideal cathode materialfor achieving a higher energy density compared to current commercialmaterials, in the desired high end portable and automotive applications.

1. A lithium metal oxide powder for a cathode material in a rechargeablebattery, comprising a core and a surface layer, the surface layer beingdelimited by an outer and an inner interface, the inner interface beingin contact with the core, the cathode material having a layered crystalstructure comprising the elements Li, M and oxygen, wherein M has theformula M=(Ni_(z)(Ni_(1/2) Mn_(1/2))_(y) Co_(x))_(1-k) A_(k), with0.15≤x≤0.30, 0.20≤z≤0.55, x+y+z=1 and 0<k≤0.1, wherein the Li content isstoichiometrically controlled with a molar ratio 0.95≤Li:M≤1.10; whereinA is at least one dopant and comprises Al, wherein the core at the innerinterface has an A1 content of 0.3-3 mol %; wherein the surface layercomprises an intimate mixture of Ni, Co, Mn, LiF and Al₂O₃ determined byXPS; and wherein the surface layer has a Mn content that decreases fromthe Mn content at the inner interface, to less than 50% of the Mncontent at the outer interface, wherein x, y, z, and k are measured byICP and Mn contents at the inner and outer interfaces are measured byXPS depth profile.
 2. The lithium metal oxide powder of claim 1, whereinthe surface layer has a Ni content that decreases from the Ni content ofthe core at the inner interface, to less than 25% of the Ni content ofthe core at the outer interface, as determined by XPS.
 3. The lithiummetal oxide powder of claim 1, wherein the surface layer has a Cocontent that decreases from the Co content of the core at the innerinterface, to less than 35% of the Co content of the core at the outerinterface, as determined by XPS.
 4. The lithium metal oxide powder ofclaim 1, wherein the surface layer further comprises one or morecompounds from the group consisting of CaO, TiO₂, MgO, WO₃, ZrO₂, Cr₂O₃and V₂O₅.
 5. The lithium metal oxide powder of claim 1, wherein thesurface layer consists of a mixture of Ni, Co and Mn and either LiF andnanometric crystalline Al₂O₃ or nanometric crystalline Al₂O₃ andsub-micrometric CaO.
 6. The lithium metal oxide powder of claim 1,wherein either 0.20≤z≤0.55 or 0.15≤x≤0.20, 0.40≤z≤0.55 and1.00≤Li:M≤1.10.
 7. The lithium metal oxide powder of claim 6, wherein0.005≤k≤0.02 and either A=Al or A=Al and Ca.
 8. The lithium metal oxidepowder of claim 1, wherein k=0.01±0.005, x=0.20±0.02, y=0.40±0.05,z=0.40±0.05, 1.00≤Li:M≤1.10 and either A=Al or A=Al and Ca.
 9. Thelithium metal oxide powder of claim 1, wherein the thickness of thesurface layer is more than 50 nm and less than 400 nm.
 10. The lithiummetal oxide powder of claim 1, wherein the F content of the core=0 mol%.
 11. A method for making the lithium metal oxide powder of claim 1,comprising: providing a first mixture comprising a lithium M′-oxidepowder, with M′=Ni_(z)(Ni_(1/2) Mn_(1/2))_(y) Co_(x), 0.15≤x≤0.30,0.10≤z≤0.55 and x+y+z=1, and a first source of A comprising Al, heatingthe first mixture to a first sintering temperature of at least 500° C.,sintering the first mixture at the first sintering temperature for afirst period of time, cooling the first sintered mixture, adding afluorine-containing polymer and a second source of A comprising Al tothe first sintered mixture, thereby obtaining a second mixture, heatingthe second mixture to a second sintering temperature between 250° and500° C., sintering the second mixture at the second sinteringtemperature for a second period of time, thereby obtaining the lithiummetal oxide powder, and cooling the powder.
 12. The method according toclaim 11, wherein one or both of the first and the second source of A isAl₂O₃.
 13. The method according to claim 12, wherein one or both of thefirst and the second source of A further comprises one or more compoundsselected from the group consisting of CaO, TiO₂, MgO, WO₃, ZrO₂, Cr₂O₃and V₂O₅.
 14. The method according to claim 11, wherein the source of Acomprises a nanometric alumina powder having a D50<100 nm and a BET≥50m²/g.
 15. The method according to claim 11, wherein the amount offluorine-containing polymer in the second mixture is between 0.1 and 2wt %.
 16. The method according to claim 11, wherein thefluorine-containing polymer comprises a PVDF homopolymer, a PVDFcopolymer, a PVDF-hexafluoropropylene (HFP) polymer or a PTFE polymer.17. An electrochemical cell comprising the lithium metal oxide powder ofclaim 1.